1. Field of the Invention
The present invention relates to seat-weight sensors, and more particularly, to seat-weight sensors for sensing a load of an occupant sitting on a vehicle seat.
2. Related Art
An airbag system as a restraint device is mounted, for example, on a driver's seat of an automotive vehicle. The airbag system includes an airbag wherein when undergoing a strong impact by a frontal collision; the airbag is inflated instantaneously to restrain a driver. In recent years, the airbag system is also mounted on a passenger's seat so as to restrain a passenger or occupant sitting on the passenger's seat from a collision impact.
A seat-weight sensor is mounted on the passenger's seat so as to sense the presence or absence of an occupant and a load thereof. The airbag system is controlled in accordance with a detection signal of the seat-weight sensor. The airbag system is controlled, for example, such that when a given load is sensed, the airbag is activated, whereas when the sensed load is smaller than the given load, the airbag is not activated.
The seat-weight sensor is arranged, for example, in each of four corners of the seat. The seat-weight sensor is formed of a rectangular sensor plate having a distortion gauge in the constricted center portion. The sensor plate has one end fixed to a seat frame for holding the seat and the other end fixed to a slide rail (upper slide rail). A load of the occupant sitting on the seat causes a deflection of the sensor plate, the amount of deflection being converted into an electric signal by a resistance-type distortion gauge, sensing the load of the occupant. In this connection, refer to U.S. Pat. No. 6,161,891 issued to Blakesley (Patent document 1).
According to the invention of Patent document 1, the weight or load of the occupant sitting on the seat can be estimated by totaling detection values of four seat-weight sensors, for example.
The seat-weight sensor disclosed in Patent document 1and formed of a rectangular sensor plate having a distortion gauge needs a rolling restraining member and a lot of coupling members, raising a problem of complicated structure. In recent years, seat-weight sensors of simple structure devoted solely to vehicle seats have been developed; in which a seat frame and an upper slide rail are coupled directly to allow sensing of a vertical load of the seat only. Such seat-weight sensors are referred to, for example, as load cells (compression-type load cells). Hereafter, the seat-weight sensor is referred to as a load cell.